1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of reducing an unnecessary toner deposition onto a recording sheet.
2. Discussion of the Background
In image forming apparatuses such as copying machines, printers, facsimile machines, or the like using an electrophotographic method, a contact type development mechanism and a roller for image transferring have commonly been employed. It has accordingly been an important factor for an image forming apparatus to assure that an unnecessary toner deposition onto a recording sheet is avoided. As one exemplary technique, a cleaning method for cleaning a transfer member of an image forming apparatus is described in Japanese Laid-Open Patent Publication No. JPAP4-120577 (1992). The image forming apparatus described in this publication includes a contact type development mechanism and a transfer member including a foam roller and performs a cleaning operation for cleaning a transfer roller. In the cleaning operation, a development operation performed by a development roller is inhibited by applying a voltage to the development roller for a predetermined time period. In this process, the voltage has a reverse polarity relative to that of a voltage normally used. Then, a transfer roller is applied with a voltage having a reverse polarity relative to that of a voltage normally used during a normal transfer operation. In this case, a polarity of the applied voltage is same as that of a normal toner charge. Thus, the transfer roller charged with the same polarity of that of the toner deposited on the transfer roller and, therefore, the toner repels against the transfer roller. As a result, the toner is cleaned off the transfer roller.
There are some more known methods for cleaning the transfer roller. For example, in one method, the transfer roller is applied with a voltage having the same polarity of the toner's normal charge so that the toner deposition onto the transfer roller can be inhibited. In this case, the toner is the normal-charged toner developed in an outside of a recording sheet area or remaining on a photoconductive member due to a defective transportation of a recording sheet. Another exemplary method teaches a technique for pressing unnecessary toner deposited on the transfer roller back to the photoconductive member during the time before the deposited toner loses its charge.
In general, the transfer roller is prone to increase its resistance by the deposition of particles including the toner and dust. When the resistance is increased to a certain level, an electric field of a transfer region formed by the photoconductive member, the transfer sheet, and the transfer roller is drastically weakened. As a result, the transfer roller cannot attract the toner forming an image on the photoconductive member, thereby deteriorates its performance of the image transfer operation.
Since the transfer roller may not instantly cause wear and has relatively small variations of resistance over time, the life time of the transfer roller is mainly bound by the above-mentioned resistance increased by the particles of the toner and dust. Particularly, it is known the toner deposition is a greater factor. If a transfer roller is not exchangeable in an image forming apparatus, the shorter life time of the transfer roller is a critical problem for the user.
In recent years, the above-mentioned image forming apparatus including the contact type development mechanism and the transfer member including the foam roller or including the contact type charging mechanism, the contact type development mechanism and the transfer member including the foam roller has strongly been needed from the recent trends for a small size, a low cost, a maintenance-free machine, a small ozone emission, etc. On the other hand, this type of image forming apparatus involves a toner deposition problem with respect to the transfer roller. More specifically, in such an image forming apparatus, toner charged in a reverse-polarity exists besides the toner charged in a normal polarity. These reverse-charged toner particles are eventually transferred onto the transfer roller and accelerates the toner deposition.
In the development method causing the development roller to make contact with the photoconductive member, the toner receives a relatively great electrostatic potential in a relatively small gap formed between the photoconductive member and the nip area of the development roller and will receive charges from the photoconductive member and/or the development roller. Thereby, the charge of the toner will be unstable. This causes the reverse-charged toner.
In some cases, a blade made of metal or the like is used in the development mechanism to press the toner so as to cause friction against the toner and the toner is accordingly charged, as a result. Even in this case, it is possible that the toner will be reverse-charged.
Also, the reverse-charged toner will be produced when the development roller is subjected to a long time usage. In this case, the development roller is worn or has the deposition of small particles including toner materials and/or dust, which causes deterioration of the development function. In the deteriorating development operation, it is possible that some toner particles will be reverse-charged and they are transferred onto the photoconductive member.
Further, a combination of the contact type charging member and the contact type development has a factor to increase the reverse-charged toner. Specifically, in the process for the contact type charging member to provide a charge to the photoconductive member, the charge voltage of the photoconductive member is composed of great amplitudes from a micro-view. In the case of using the contact type development mechanism, the reverse-charged toner will easily be produced by a local part of such a great amplitude or at a higher part of the charge voltage of the photoconductive member.
The deposition of the reverse-charged toner easily occurs during the normal image forming operation, the time when the photoconductive member is rotated in the interval between the recording sheets, in the machine warm-up time, in the machine cool-down time, and so on. When the transfer roller makes contact with the photoconductive member, the toner is transferred onto the transfer roller. This causes the next recording sheet to become dirty. In some cases, even a backup roller in the following fixing mechanism will be made dirty. When the toner is undesirably conveyed by the rear surface of the recording sheet onto the backup roller of the fixing mechanism, a serious problem may occur. For example, the recording sheet may be pulled into the mechanism.
Therefore, the above-mentioned image forming apparatus including the contact type development mechanism and the transfer member including the foam roller or including the contact type charging mechanism, the contact type development mechanism and the transfer member including the foam roller is needed for a countermeasure for protecting the reverse-charge toner deposition, which is not solved by the above-mentioned Japanese Laid-Open Patent Publication No. JPAP4-120577 (1992).
In addition, the development roller may possibly be reverse-charged during the charging operation relative to the photoconductive member. In this case, a greater amount of the reverse-charged toner will be produced and deposited onto the photoconductive member. Therefore, performing the cleaning operation relative to the transfer roller under such a circumstance may not be effective.